diff --git a/llvm/lib/Analysis/ScalarEvolutionExpander.cpp b/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
--- a/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
@@ -2145,13 +2145,20 @@
   // If we can find an existing value for this scev available at the point "At"
   // then consider the expression cheap.
   if (At && getRelatedExistingExpansion(S, At, L))
-    return false;
+    return false; // Consider the expression to be free.
 
-  // Zero/One operand expressions
   switch (S->getSCEVType()) {
   case scUnknown:
   case scConstant:
-    return false;
+    return false; // Assume to be zero-cost.
+  }
+
+  // The rest of the logic is recursive!
+  if (!TTI)
+    return true; // No cost model - give up.
+
+  // Zero/One operand expressions
+  switch (S->getSCEVType()) {
   case scTruncate:
     return isHighCostExpansionHelper(cast<SCEVTruncateExpr>(S)->getOperand(), L,
                                      At, BudgetRemaining, TTI, Processed);
@@ -2163,7 +2170,6 @@
                                      L, At, BudgetRemaining, TTI, Processed);
   }
 
-
   if (auto *UDivExpr = dyn_cast<SCEVUDivExpr>(S)) {
     // If the divisor is a power of two and the SCEV type fits in a native
     // integer (and the LHS not expensive), consider the division cheap